In U.S. Pat. No. 3,662,140 issued May 9, 1972 to William C. Jones and Dan R. McConnell, there is disclosed a system for protecting a magnetron in a microwave oven with an isolator which uses a three-port circulator. However, it has been discovered that in commercial food merchandising a large number of food products produce large amounts of reflected energy toward the isolator from the oven and, hence, the oven is frequently de-energized due to overheating of the isolator and during the cool-down period of the isolator, the oven is not operable. Such cooling time, which may be fifteen minutes to a half hour during the rush hour of a food merchandising establishment, is costly and can lead to loss of business.